How long is a piece of string, your budget will depend on numerous thing’s, as you haven’t mentioned anything, as where you are staying, what you like to do etc etc, impossible to say, but allow at least a minimum of 75,000 baht person per month
health insurance is mandatory if you are on a OA visa, but otherwise it is entirely up to you, but I would recommend you do. Again any cover you choose would be based on your age, preexisting conditions, deductible, what level you require, your budget etc.
It depends on so many things if you are a bit more specific you might get some better answers. You can try AXA or Pacific Cross for starters, but way to many variables to answer a generic question.